1. Field of the Invention
The invention relates generally to an output buffer, and more particularly to an output buffer having two output stages.
2. Description of Related Art
The source driver is an important element in a driving system of a display apparatus, which is used for providing a driving voltage to a pixel electrode corresponding to an enabled scan line. Due to a panel loading effect and manufacturing variables, the driving voltage provided to the pixel electrode is usually less than ideal. Therefore, the source driver needs an output buffer to manipulate the driving voltage to a specific voltage level. Moreover, as applications of large size panels and frequency multiplying techniques emerge, the output buffer is required to bear an enormous load. Accordingly, for rapid driving the source driver needs to utilize an output buffer having a high slew rate or a high driving ability.
In order to increase the slew rate of the output buffer, U.S. Pat. No. 7,265,593 discloses a “Slew rate enhancement circuit via dynamic output stage for adjusting gamma curve”. The output stage disclosed in this patent includes an amplifier, a main output stage, a monitoring control device, and an auxiliary output stage. The main output stage is controlled by the amplifier, and accordingly the main output stage generates a boost signal and a pull signal. The monitoring control device is used for weakening the boost signal and the pull signal. The weakened boost signal and the pull signal are used to control the auxiliary output stage. Accordingly, when the input signal is not equal to the output signal, not only the main output stage but also the auxiliary output stage can provide a current to the load. However, when the input signal is equal to the output signal, the monitoring control device must continually turn off the auxiliary output stage. In other words, under a stable system, the monitoring control device must consume a specific amount of static current to turn off the auxiliary output stage, thereby increasing the power consumption of the output buffer.
In addition, U.S. Pat. No. 7,274,254 discloses a “Device and method for enhancing output current driving”. The output stage disclosed in this patent includes an operational amplifier, an output stage, an auxiliary output device, and a monitor device. The output stage is controlled by the operational amplifier. The monitor device is for monitoring the input status of the operational amplifier, thereby controlling the auxiliary output device. Accordingly, when the input signal is not equal to the output signal, the auxiliary output device can increase the driving ability of the output device. However, when the input signal is the same as the output signal, the monitor device must continually consume a static current to continually turn off the auxiliary output device. In other words, the monitor device not only increases the power consumption of the overall system, but the addition of the monitor device also enlarges the layout area of the system.